The present invention relates to an electrical filter and a signal discriminating apparatus employing the electrical filter for discriminating two signals having frequencies different from each other.
Commonly, in electrical and electronic systems, there are cases where it is required to discriminate whether either of two signals having frequencies different from each other is present or neither of these two signals is present.
In the sound multiplex system in the television broadcasting, for example, when a stand-in voice reproducing device is to be operated, a signal of 922.5 Hz as a control signal is placed upon the carrier signal of 55.125 MHz. Meanwhile, when a stereo-broadcast reproducing device is to be operated, a signal of 982.5 Hz in frequency as a control signal may be placed upon said carrier signal.
Conventionally, as shown in FIG. 1, a piezo-electric tuning fork 2 with a central frequency of 922.5 Hz and a piezo-electric tuning fork 3 with a central frequency of 982.5 Hz are connected in parallel to the output side of the amplifier 1 to amplify the output of the tuning fork 2 by a amplifier 4 thereby to operate a relay 5, while the output of the tuning fork 3 is amplified by an amplifier 6 to operate a relay 7. Accordingly, when neither the signal of 922.5 Hz nor the signal of 982.5 Hz is placed on the carrier signal, neither the relay 5 nor the relay 7 operates. Thus, neither the stand-in voice reproducing device 8 nor the stereo-broadcast reproducing device 9 operates. When the signal of 922.5 Hz is placed upon the carrier signal, output is developed only at the output side of the tuning fork 2 to operate the relay 5, whereby the stand-in voice reproducing device 8 functions. On the other hand, when the signal of 982.5 Hz is placed upon the carrier signal, output is produced only at the output side of the tuning fork 3 to operate the relay 7, whereby the stereo-broadcast reproducing device 9 functions.
In the known arrangement as described hereinabove, tuning forks equal in number to the control signals are required, thus resulting in high cost. Furthermore, since the control signals are close to each other in frequency, separating means having superior selectivity, for example, piezo-electric tuning forks and the like, are required instead of inexpensive LC filters, thus making it difficult to reduce the manufacturing cost. For developing any new systems for wide application as well as for said sound multiplex system described above, it is indispensably required to provide necessary apparatuses at low cost as far as possible from manufacturing point of view.
Additionally, in the conventional arrangement as described in the foregoing, signal level of the central frequency of the tuning fork 2 or 3 is important. More specifically, even when signals of the central frequencies of the respective tuning forks 2 and 3 are present, the relays 5 and 7 are not actuated if these signals are below predetermined levels, while sufficient amplification of the signals may result in malfunctions of the relays 5 and 7 due to internal and external noise, etc. Moreover, it is required to employ filters which are superior in selectivity due to proximity in frequency of the two signals, but extremely high selectivity tends to give rise to variations in characteristics of the filter, e.g., the temperature characteristics thereof, due to fluctuation in the external circumstances of the filter, counter-measures against which are difficult.